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ORMs in Node.js: Sequelize & Mongoose

Introduction to ORMs

Object-Relational Mapping (ORM) is a technique that allows developers to interact with a database using an object-oriented paradigm. It abstracts the database interactions, making it easier to work with data by representing tables as classes and rows as objects.

Sequelize

Sequelize is a promise-based ORM for Node.js, supporting various SQL dialects like PostgreSQL, MySQL, SQLite, and MSSQL. It provides a robust set of features for managing database models, associations, and transactions.

Mongoose

Mongoose is an ODM (Object Data Modeling) library for MongoDB and Node.js. It provides a schema-based solution to model your application data, offering validation, type casting, and built-in query building capabilities.

Setting up Sequelize

To set up Sequelize, you need to install it along with the necessary database driver. For example, for PostgreSQL, you'd use:


npm install sequelize pg pg-hstore

Database Configuration

After installation, configure Sequelize by creating a new instance with your database credentials.


const { Sequelize } = require('sequelize');
const sequelize = new Sequelize('database', 'username', 'password', {
  host: 'localhost',
  dialect: 'postgres'
});

Testing Connection

Test the database connection to ensure everything is set up correctly.


sequelize.authenticate()
  .then(() => {
    console.log('Connection has been established successfully.');
  })
  .catch(err => {
    console.error('Unable to connect to the database:', err);
  });

Defining Models in Sequelize

Models in Sequelize are defined using the `define` method. Each model represents a table in the database.


const User = sequelize.define('User', {
  firstName: {
    type: Sequelize.STRING,
    allowNull: false
  },
  lastName: {
    type: Sequelize.STRING
  }
});

Synchronizing Models

To create the tables defined by your models in the database, use the `sync` method.


sequelize.sync()
  .then(() => {
    console.log('Database & tables created!');
  });

Setting up Mongoose

To set up Mongoose, you need to install it via npm and then connect to your MongoDB database.


npm install mongoose

Connecting to MongoDB

Use the `connect` method to establish a connection to your MongoDB database.


const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/my_database', {useNewUrlParser: true, useUnifiedTopology: true});

Defining Schemas in Mongoose

Schemas in Mongoose define the structure of documents within a collection.


const userSchema = new mongoose.Schema({
  name: String,
  age: Number,
  email: String
});

Creating Models

Once a schema is defined, it can be converted into a Model which represents a collection in the database.


const User = mongoose.model('User', userSchema);

Querying Data with Mongoose

Mongoose provides a variety of methods for querying data, such as `find`, `findOne`, and `findById`.


User.find({ name: 'John' }, function (err, users) {
  if (err) return console.error(err);
  console.log(users);
});

Associations in Sequelize

Sequelize supports various types of associations like one-to-one, one-to-many, and many-to-many. These associations help in defining relationships between models.


const Project = sequelize.define('Project', {/* attributes */});
const Task = sequelize.define('Task', {/* attributes */});

Project.hasMany(Task, {as: 'Tasks'});
Task.belongsTo(Project);

Validation in Mongoose

Mongoose provides built-in validation for schema types. You can also define custom validation logic to enforce rules on your data.


const userSchema = new mongoose.Schema({
  email: {
    type: String,
    required: [true, 'Email is required'],
    match: [/^\S+@\S+\.\S+$/, 'Please use a valid email address']
  }
});